Production of electronic circuits, in the form of very large scale integrated (VLSI) circuits, has recently become common place. A typical VLSI circuit is usually of the digital circuit type, and of such complexity that may be more appropriately referred to as a system. However, in some circumstances a complete practical system is not attainable in a single VLSI circuit of the digital circuit type, because of a requirement to interface with external analog signals. Such an interface requirement is usually provided by an analog-to-digital conversion circuit or by a digital-to-analog conversion circuit as the case requires. It is usual that some analog circuitry is required for coupling between analog signal ports and the conversion circuitry.
Among various VLSI manufacturing technologies, those well suited for producing digital VLSI circuit elements are not well suited to providing analog circuit elements. For example the operating characteristics of an analog filter circuit are determined by circuit component values such as resistance and reactance. However, in a VLSI digital optimized technology, such as a complementary metal oxide semiconductor (CMOS) technology, the values of resistive and capactive circuit elements are not nearly as precisely obtainable as with discretely manufactured or selectively trimmed components. Furthermore active analog elements such as differential amplifiers and the like, provided by this technology, have characteristics which are inferior as compared with typical off-the-shelf active analog components.
In spite of these difficulties analog signal circuits have been provided in CMOS technology VLSI circuits, albeit with compromised but tolerable functionality. This has been achieved by a filter design which is optimized to be as insensitive as possible to processing and temperature variation. In this design, distributed rather than lumped capacitive elements are used. The distributed capacitive element is formed along a resistive path of an associated resistive element to provide a resistor capacitor pair configuration. A conductive electrode provides a plate of the capacitor and carries a dielectric layer between it and a polycrystalline silicon layer which is tailored to provide the resistor element. This produces a distributed capacitive effect and reduces stray capacitance effects between the resistor element and the substrate of the VLSI circuit. The product of the R and C values of the pair configuration remains somewhat stable in spite of undesirable process variations. For example if an over etch variation occurs a thinner resistor element is produced having an increased resistance value. At the same time however, the capacitance coupling at the conductive electrode is decreased. Since the pole frequency of the filter is determined by the RC products of the pair configuration, the process variation tends to be of little consequence as compared to a circuit with independent capacitor and resistor elements.
In one example, a simple network of paired resistive and capacitive elements is connected with a differential amplifier to provide a second order low pass filter for analog signals entering a VLSI circuit. Such a filter exhibits all the required low passband filter characteristics except for higher frequencies which are about an octave or more removed from the passband. Signals of such frequencies tend to traverse the analog filter with less than desirable attenuation and tend to have a deleterious effect upon any subsequent analog-to-digital conversion function.